1. Field of the Invention
The present invention relates to a bottom gate thin film transistor (hereinafter, abbreviated as a TFT) in which an amorphous semiconductor film provided over a substrate is used, a circuit formed by the TFT, a device including the circuit formed by the TFT, and a manufacturing method thereof.
In particular, the present invention relates to an electrooptic device typified by a liquid crystal display device and a technique which can be favorably applied to an electronic appliance provided with such an electrooptic device.
2. Description of the Related Art
In recent years, a direct-view liquid crystal display device is widely used in order to display an image or character information on an electronic appliance such as a monitor of a laptop personal computer or a desktop personal computer, a mobile phone, an audio reproducing device, a television device, a mobile terminal, a digital still camera, a video camera, or a viewer for viewing an image and a moving picture.
In particular, an active matrix liquid crystal display device enables a high-definition image in comparison with a passive liquid crystal display device, thereby being widely used.
Active elements (for example, thin film transistors) are arranged corresponding to pixels respectively in matrix in a pixel portion which is to be a display region, thereby constituting the active matrix liquid crystal display device. A TFT, as a switching element, controls voltage which is applied to liquid crystal in each pixel so that desirable image display is performed (see Patent Document 1: Japanese Published Patent Application No. 2002-116712).
In the active matrix liquid crystal display device, a TFT, a wiring, an electrode, a contact hole in an insulating film, and the like are formed over a substrate by using a plurality of photomaks by a photolithography technique.
When the wiring or the electrode is formed using metal such as aluminum (Al), tungsten (W), or titanium (Ti), a desired pattern can be formed by performing either dry etching or wet etching.
In addition, as for a light-transmitting conductive film (also referred to as a “transparent conductive film” in this specification) which is used as a material for a pixel electrode of a transmissive liquid crystal display device, a desired pattern can be formed by performing either dry etching or wet etching.
As such a transparent conductive film, metal oxide such as indium tin oxide (hereinafter, also referred to as “ITO”), zinc oxide, or indium zinc oxide (hereinafter, also referred to as “IZO”), or semiconductor oxide is used.
In particular, a transparent conductive film is etched mainly by wet etching.
However, the transparent conductive film exemplified above is disadvantageous in that a residue is easily generated in comparison with metal such as aluminum (Al). Therefore, when a residue is generated and remains over a substrate eventually, current leak might be caused between pixel electrodes.
As is the case with the above transparent conductive film, an insulating film such as a silicon nitride film or a silicon oxide film is disadvantageous in that a residue generated due to wet etching remains in a connection portion between conductive materials. Therefore, there is concern that contact defect or increase in contact resistance might be caused.
In a liquid crystal display device in which a conventional TFT is used, a semiconductor film which becomes a core of a switching function or the whole TFT is covered with a protective film (also referred to as a “passivation film”) formed of a silicon nitride film, a silicon oxide film containing nitrogen, or a silicon nitride film containing oxygen in order to be protected from contamination.
The contamination here means alkali metal such as lithium (Li), sodium (Na), or potassium (K), which has an effect of deteriorating a function of a semiconductor as switching.
However, in a case of a transmissive liquid crystal display device or a semi-transmissive liquid crystal display device, this protective film is formed in an aperture which transmits light of backlight and forms an image to be displayed, as well as in an area where a TFT is formed.
Although the light of the backlight penetrates the protective film in the aperture, final intensity of transmitted light is decreased because the light is reflected, refracted, absorbed, or the like inside the protective film. Therefore, luminance of the liquid crystal display device may be decreased compared to that of a backlight source. Moreover, a wavelength of the light is changed from that of the light source after penetrating the protective film for the same reason, and a difference between a color displayed practically and an intended color may be caused.
The conventional active matrix liquid crystal display device includes pixels arranged in matrix, and a method of displaying an image in a display region (a pixel portion) by selecting a scanning line (a gate wiring) by a line sequential driving method is mainly used.
Each scanning signal line is selected in a cycle of 60 Hz or the like. However, an auxiliary capacitor (Cs) is provided in each pixel in order to hold the electric potential of a pixel electrode during a period between the termination of writing to an arbitrary row and the start of writing in the next cycle.
Two methods can be considered as a formation method of the auxiliary capacitor in a conventional active matrix liquid crystal display device in which an amorphous semiconductor film is used: a method in which an auxiliary capacitor is formed with a wiring formed of the same material and formed in the same layer as those of a gate wiring (a scanning line) or gate wiring of the adjacent pixel row as one of the electrodes, a pixel electrode as the other electrode, and a gate insulating film and a protective film interposed between the two electrodes (hereinafter, referred to as a “first method”); and a method in which an auxiliary capacitor is formed with an auxiliary capacitor wiring formed of the same material in the same layer as those of the gate wiring and which is separated from a gate wiring, as one of electrodes, an electrode formed of the same material in the same layer as those of a drain electrode and connected to a pixel electrode as the other electrode, and a gate insulating film interposed therebetween (hereinafter, referred to as a “second method”).
A top view of a pixel of a conventional active matrix liquid crystal display device is shown in FIG. 2. The liquid crystal display device shown in FIG. 2 includes a gate electrode and gate wiring (the “gate wiring” is also referred to as a “scanning line”) 1002, a semiconductor film 1003 of a TFT, a source electrode and source wiring (the “source wiring” is also referred to as a “signal line”) 1004, a drain electrode 1005, a pixel electrode 1006, and an auxiliary capacitor 1007. The auxiliary capacitor 1007 is formed of the gate wiring 1002, the pixel electrode 1006, and an insulating film (a dielectric film) formed between the gate wiring 1002 and the pixel electrode 1006.
A manufacturing process of the conventional active matrix liquid crystal display device shown in FIG. 2 will be explained with reference to FIGS. 12A to 12F and FIGS. 13A to 13E. It is to be noted that FIGS. 12A to 12F and FIGS. 13A to 13E correspond to a cross section taken along a line B-B′ of FIG. 2.
First, a first conductive film 1021 is formed over a substrate 1000 (see FIG. 12A). Then, a resist mask is formed by a first photolithography process, and an unnecessary portion of the first conductive film 1021 is removed by etching, thereby forming a gate electrode and gate wiring 1002 (see FIG. 12B).
A gate insulating film 1022, an amorphous semiconductor film 1023, and an amorphous semiconductor film 1024 containing an impurity imparting one conductivity type are formed over the substrate 1000 and the gate electrode and gate wiring 1002 (see FIG. 12C). Then, a resist mask is formed by a second photolithography process and unnecessary portions of the amorphous semiconductor film 1023 and the amorphous semiconductor film 1024 containing an impurity imparting one conductivity type are removed by etching, thereby forming an island-shaped semiconductor film 1025a and an island-shaped semiconductor film containing an impurity 1025b (see FIG. 12D).
Next, a second conductive film 1026 is formed over the gate insulating film 1022, the island-shaped semiconductor film 1025a, and the island-shaped semiconductor film containing an impurity 1025b (see FIG. 12E). Then, a resist mask is formed by a third photolithography process, and an unnecessary portion of the second conductive film 1026 is etched, thereby forming a source electrode and source wiring 1004 and a drain electrode 1005 (see FIG. 12F).
Then, the island-shaped semiconductor film 1025a and the island-shaped semiconductor film containing an impurity 1025b are etched in self-alignment manner by using the source electrode and source wiring 1004 and the drain electrode 1005 as masks. The island-shaped semiconductor film containing an impurity 1025b is divided into a source region 1003bs and a drain region 1003bd. Also, the island-shaped semiconductor film 1025a is etched to be an island-shaped semiconductor film 1003a (see FIG. 13A).
A protective film 1027 is formed over the source electrode and source wiring 1004, the drain electrode 1005, the source region 1003bs, the drain region 1003bd, and the island-shaped semiconductor film 1003a (see FIG. 13B). A resist mask is formed by a fourth photolithography process, and the protective film 1027 is etched, thereby forming a contact hole 1001 reaching the drain electrode 1005 (see FIG. 13C).
A third conductive film 1029 is formed covering the protective film 1027 and the contact hole 1001 (see FIG. 13D). A resist mask is formed by a fifth photolithography process, and the third conductive film 1029 is etched, thereby forming a pixel electrode 1006 (see FIG. 13E).
As descried above, the pixel of the conventional active matrix liquid crystal display device shown in FIG. 2 is formed by five photolithography processes with the use of the five photomasks.
An example in which an auxiliary capacitor is formed by the above first method is shown in FIG. 2 and FIG. 11. The liquid crystal display device shown in FIG. 2 is an example in which the gate wiring (scanning line) of the adjacent pixel row is one of electrodes and a pixel electrode is the other electrode. In FIG. 2, reference numeral 1002 denotes a gate electrode and gate wiring; 1003, a semiconductor film of a TFT; 1004, a source electrode and source wiring; 1005, a drain electrode; and 1006, a pixel electrode. An auxiliary capacitor 1007 is formed by using the gate wiring 1002, the pixel electrode 1006, a gate insulating film and a protective film formed between the gate wiring 1002 and the pixel electrode 1006 as dielectric films.
A liquid crystal display device shown in FIG. 11 is an example in which a wiring formed of the same material and formed in the same layer as those of a gate wiring is one of electrodes and a pixel electrode is the other electrode. In FIG. 11, reference numeral 1012 denotes a gate electrode and gate wiring; 1013, a semiconductor film of a TFT; 1014, a source electrode and source wiring; 1015, a drain electrode; 1016, a pixel electrode; 1017, an auxiliary capacitor; and 1018, an auxiliary capacitor wiring. The drain electrode 1015 and the pixel electrode 1016 are connected to each other through a contact hole 1011. The auxiliary capacitor wiring 1018 is formed of the same material and formed in the same layer as those of the gate electrode and gate wiring 1012.
The auxiliary capacitor 1017 is formed by using the auxiliary capacitor wiring 1018, the pixel electrode 1016, and a gate insulating film and a protective film formed between the auxiliary capacitor wiring 1018 and the pixel electrode 1016 as dielectric films.
An example in which an auxiliary capacitor is formed by the second method is shown in FIG. 50. In FIG. 50, reference numeral 1032 denotes a gate electrode and gate wiring; 1033, a semiconductor film of a TFT; 1034, a source electrode and source wiring; 1035, a drain electrode; 1036, a pixel electrode; 1037a and 1037b, auxiliary capacitors; 1038, a lower auxiliary capacitor wiring; and 1039a and 1039b, upper auxiliary capacitor electrodes. The drain electrode 1035 is connected to the pixel electrode 1036 through a contact hole 1031.
The upper auxiliary capacitor electrode 1039a is formed of the same material and formed in the same layer as those of the source electrode and source wiring 1034 and the drain electrode 1035, and is connected to the pixel electrode 1036 through two contact holes. The auxiliary capacitor 1037a is formed using the lower auxiliary capacitor wiring 1038 as one of the electrodes, the upper auxiliary electrode 1039a as the other electrode, and a gate insulating film as a dielectric body between the electrodes.
The upper auxiliary capacitor electrode 1039b is also formed of the same material and formed in the same layer as those of the source electrode and source wiring 1034 and the drain electrode 1035, and is connected to the pixel electrode 1036 through a contact hole. The auxiliary capacitor 1037b is formed by using the lower auxiliary capacitor wiring 1038 as one of the electrodes, the upper auxiliary capacitor electrode 1039b as the other electrode, and the gate insulating film as a dielectric body between the electrodes.
Since the dielectric film between the two electrodes may be as thick as one gate insulating film in the second method, the film thickness of the dielectric film can be reduced; accordingly, capacitance can be increased. Therefore, an area for forming the auxiliary capacitor necessary for the second method is smaller than that of the first method.
However, in the second method, there are problems in that an opening ratio is decreased because an area for forming the electrode is necessary in order to provide the auxiliary capacitor line as one of the electrodes in addition to the scanning line (the gate wiring) and yield is decreased because the electrode formed of the same material and formed in the same layer as those of the drain electrode is formed as the other electrode.
On the other hand, in the first method, capacitance is smaller than that of the second method because two layers that are the gate insulating film and the protective film are used for the dielectric films; therefore, a larger area for forming the electrode is necessary. For this reason, problems in that the scanning line itself has to be wide and a portion of the scanning line overlapping with the pixel electrode has to be taken large in design occur.
Furthermore, as shown in FIG. 2 and FIG. 11, in the conventional active matrix liquid crystal display device, the source electrode or the drain electrode connected to the TFT and the pixel electrode are connected to each other through the circular contact hole 1001 or 1011. Accordingly, the TFT and the pixel electrode are electrically connected to each other.
However, a concave is caused on an alignment film due to the contact hole under the alignment film; therefore, ideal rubbing is difficult. For this reason, there is a disadvantage in that disclination of liquid crystals arranged near the portion over the contact hole occurs. Therefore, there is a problem in that quality of display could be poor because light leaks near the portion over the contact hole.
In order to prevent such a problem, there is a method of providing black matrix at a counter substrate side which faces a substrate over which a TFT is formed. In this case, light is shielded in the area where the contact hole is located and a vicinity thereof. However, this method is one of factors that decrease an aperture ratio.
The protective film with a predetermined shape is formed by dry etching of a material for forming the protective film. At this time, an unnecessary substance such as part of the protective film material or a reaction product of the material for forming the protective film and an etching gas component which is generated due to etching remains on a surface to be processed as a residue. For example, when this residue is generated between the wiring and the pixel electrode, the residue can function as contact resistance between the pixel electrode and the wiring or prevent electrical contact. Eventually, the residue can seriously damage a function as a liquid crystal display device or make the liquid crystal display impossible to work out.
Therefore, in order to prevent the residue from remaining, the surface to be processed is cleaned with a fluorine-based chemical, alkali cleaner, a surface active agent, or pure water or a combination of these and ultrasonic cleaning (hereinafter, referred to as cleaner).
However, although, in a structure of the conventional liquid crystal display device, a circular contact hole with a small diameter is used in a contact portion between the pixel electrode and the drain electrode, it has been concerned that a residue or a cleaning liquid might remain on the inner wall or at the bottom of the circular contact hole when a substrate with a surface to be processed is drawn from cleaner after cleaning.
In addition, it has been concerned that, with such a conventional contact hole with a small diameter, the pixel electrode and the drain electrode could be disconnected due to a bump and poor connection might occur.
As a liquid crystal display device, there are a transmissive liquid crystal display device in which light from a backlight provided at the rear of a display device penetrates and display is performed and a reflective liquid crystal display device in which external light is reflected by a reflective electrode provided over a substrate and display is performed.
The transmissive liquid crystal display device is superior in visibility also in a dark indoor place or the like, and the reflective liquid crystal display device is superior in visibility in a bright place of outside. For a display device used both inside and outside, like a cellular phone, there is a semi-transmissive (including a transmissive region and a reflective region approximately equally) liquid crystal display device with both functions of the transmssive liquid crystal display device and the reflective liquid crystal display device, or micro reflective (a reflective region is smaller than a transmissive region) liquid crystal display device.